The applicant is requesting five years of funding through the Patient- Oriented Research Career Award (K-23) program to enhance her independence as an investigator. The proposed training is in crucial basic methods for research in clinical and psychopharmacology of children with Attention Deficit Hyperactivity Disorder (ADHD). Methylphenidate (Ritalin(R)) is the most frequently prescribed psychostimulant by physicians to treat cases of ADHD. Few studies have been done to evaluate the efficacy of methylphenidate (MPH) in preschoolers with ADHD or to compare the time-response function of this age group with school-aged children. No study has addressed the comparative pharmacokinetics of MPH in these two age groups. The applicant will address these questions. The training will be based at the University of California, Irvine Child Development Canter, which has a well-established program for the treatment of ADHD and is a leading site for studying pharmacokinetics (PK) and pharmacodynamics (PD) of psychoactive stimulants. A unique component is the laboratory school paradigm (LSP) which was developed here. The research plan is designed to evaluate the variability of PD of MPH in preschoolers (3 to 5 year olds) versus school-aged (6 to 8 year olds) children with ADHD. Within-subject PK variability in time-course of serum concentration for each medication will be analyzed, and the time-response characteristics also will be estimated based on PD measures for methylphenidate.